For amplifiers including, for example, a power amplifier used for a communication apparatus including a mobile phone, a radio-base station, and so forth, the linearity has been demanded to amplify an input signal (e.g., a transmission signal) with low distortion. For achieving the linearity of an amplifier, remedies for compensating for a nonlinear distortion (nonlinear-waveform distortion) occurring in the amplifier have been performed. As an example, a negative-feedback system has been available as an exemplary nonlinear-distortion compensation system.
It has been recognized that a delay occurs in a negative-feedback loop due to the substrate wiring, a circuit component for use, the circuit configuration, and so forth when compensating for the nonlinear distortion through the use of the negative-feedback system. The above-described delay causes a phase shift in the negative-feedback loop, the phase shift corresponding to the frequency of the input signal. It is assumed that a signal component with a phase shift of 180° or more is negatively fed back to the input of the amplifier at that time. However, the negative feedback of the signal component with a phase shift of 180° or more actually becomes a positive feedback for the input signal, which is undesirable because the positive feedback for the input signal causes the amplifier to oscillate. Therefore, a filter provided to set the gain of the signal component with a phase shift of 180° or more to 0 dB or less may be provided in the negative-feedback loop considering the delay in the negative-feedback loop.
Hitherto, the design of the filter provided in the negative-feedback loop (e.g., the design of the band of the filter) has been performed through a maker manufacturing the filter and/or an amplifier including the filter by following the steps below. First, the property of the negative-feedback loop including no filter is measured. After that, the band of the filter is adjusted as desired to set the gain of a signal component having a phase sift of 180° or more, which occurs in the negative-feedback loop, to 0 dB or less by providing the filter in the negative-feedback loop. Thus, the filter design is performed.